The present invention relates to an improved X-ray detector for use in an automatic X-ray exposure unit for an X-ray radiographic device, and a method for manufacturing the X-ray detector.
In an X-ray diagnostic device, i.e., an X-ray radiographic device, X-rays transmitted through an object under examination are applied to an X-ray image-forming material, e.g., an X-ray film, to form a transmitted X-ray image. An automatic X-ray exposure unit is provided in the radiographic device and has an X-ray detector. It detects X-rays incident on the X-ray film and monitors the X-ray dose. The unit controls the X-ray exposure time, so that X-ray radiation is automatically stopped when an X-ray dose necessary for obtaining an X-ray picture of adequate density is radiated.
Among X-ray detectors used in the automatic X-ray exposure unit are: one which comprises a combination of an intensifying screen, for converting X-rays to a light beam, and an optical detector; and another which uses a semiconductor X-ray detector, and the like. The X-ray detector, that converts the X-rays to a light beam with an intensifying screen and detects the light beam with an optical detector, uses a transparent acrylic resin plate to guide light from the intensifying screen to the optical detector. More specifically, this type of X-ray detector consists of an X-ray detecting section and an optical detector. The X-ray detecting section has an acrylic resin plate, an X-ray/light-converting intensifying screen, and a light-reflecting layer. The acrylic resin plate has an irregular reflecting surface for receiving light at a predetermined detection region. The intensifying screen is arranged on a portion of one surface of the plate corresponding to the irregular reflecting surface. The reflecting layer is arranged on the other surface of the plate. A light-guide projection is formed on one side of the plate. The intensifying screen is arranged on one entire surface of the plate, with a light-shielding sheet interposed therebetween. The light-receiving section of the optical detector is arranged on the distal end face of the light-guide projection. The detector is completely covered by a light-shielding film, except for a contact section thereof.
The acrylic resin plate conventionally used in the X-ray detecting section of the X-ray detector does not have a sufficiently high transmittance. In the above X-ray detector, light is transmitted from the intensifying screen to the optical detector through the acrylic resin plate, utilizing the irregular reflection of light from the irregular reflecting portion formed thereon, resulting in poor light transmission efficiency. A photomultiplier is conventionally used as the optical detector. The photomultiplier must be arranged in tight contact with the distal end face of the light-guiding projection of the acrylic resin plate. As a result, the size of the photomultiplier defines the substantial area occupied by the X-ray detector. In other words, the size of the X-ray detector hinders size reduction and simplification of the structure of the overall device. For example, the thickness of the X-ray detecting section can be set to be about 3.0 mm, whereas the diameter of the photomultiplier is 1 to 2 cm at minimum. Therefore, the peripheral members of the X-ray detector must be arranged away from the photomultiplier, of the 1 to 2 cm or more diameter, arranged in the vicinity of the acrylic resin plate of the 3.0 mm thickness, and also away from the acrylic resin plate.
The larger the distance between the converting intensifying screen and the optical detector, the larger the attenuation in the light converted from X-rays and guided to the optical detector through the acrylic resin plate. The light detecting coefficiency of the photomultiplier within a light-receiving area corresponding to a predetermined X-ray detection area, i.e., the irregular reflecting section for receiving light from the intensifying screen, is thus non-uniform, resulting in a non-uniform X-ray detecting coefficiency within the predetermined X-ray detection area.